First off, I just wanted to thank Mr Tomcat for bravely moderating this lengthy but not so lengthy GB!
Here's a few things I've learned doing blitz builds:
Pick a kit you're familiar with. When time is tight, you don't want to have to figure things out.
Which leads to planning, as Mr Wombat said. This will save time & trouble.
Sort out decals in advance, along with replacements.
Stick to a simple color scheme (as noted by kerick). The best I've been able to manage is two-tone uppers with a third underside color.
Use CA (super glue) and acrylic paints. These dry fast enough to make a blitz a lot more achievable.
I also like painting by hand with a brush. Nothing could be simpler or take less prep and there's no take-down like with an airbrush.
Strategic snacking and tactical use of adult beverages is also helpful.
Brian da Basher